It’s possible that the glass has been etched. One more thing to try would be glass cooktop cleaner

try a few rounds of Lime-a-way spray and magic erasers before doing anything more drastic with harsher chemicals. Glass cooktop cleaner is good but will run a lot down the surface and drip.

I am a fan of the Bio-Clean, too. It's simply amazing. There are other products/brands with the name Bio-Clean, but be sure you get the one pictured that has the hyphen between Bio and Clean. I haven't used the lotion/bottle pictured, but have bought the stuff in tubs: Thei

https://www.bio-cleanproducts.com/product/buy-two-48-oz-bottles-get-one-free/

Heat a cup of vinegar in the microwave until bubbly hot, but not boiling. Add a generous squirt of dishsoap (I.e. Dawn original). Slosh onto scummy area with a rag. Wait...This is what makes it work...10 to 15 minutes. Use the rag to wipe away the scum. If it's really horrible, wipe with hot as you can stand it water, too. Make sure you turn on a fan and open a window. Hot vinegar stinks!

Try a straight edge razor to scrape it off. I use the razor on my glass cooktop for the stuff that will not come off with the cleaner.

Thanks everyone!!! I’m afraid it is etched! I just tried a razor and no luck. If it’s etched I wonder if that means it’s trashed?

I don't know if it will work, but what about that stuff used on car headlights that are etched. You just never know what will work. Just keep on trying. My brown vinyl garage door was faded and nothing I used for years would help restore the color. Just on a lark I picked up a can of tire blacking from the dollar tree. I applied it in the spring and it still looks like a brand new door. .

Go to ACE if you can and get a sample of the bio-clean. I didn't have any luck with a razor blade removing some hard water stains/build up that bio-clean dissolved in one minute. AND it seems to put some kind of protection for future build up, since the places I cleaned haven't gotten a build up again.

I use Lemi-Shine dishwasher booster. It is sold next to DW detergents. Pure citric acid so no noxious fumes. I sprinkle on a very wet sponge along with a little BKF. Works great for us, and worth a try in the industrial strength crud you've got. I use LemiShine to remove mineral buildup on faucets, refrigerator water dispenser, etc. Good luck!

I just Lime Away with a Mr. Clean Magic Eraser. It works well.

You could try CLR as well... I don't have glass doors but I use it to clean the showerhead...

Good luck!